The primary goal of this study is to evaluate what effects bile externalization and antibiotic gut sterilization have on the enterohepatic recirulation (EHC) pharmacokinetics of mycophenolic acid (MPA) in orthotopic liver transplant (OLT) patients. A secondary goal is to evaluate the relationship between MPA serum concentrations, free and total, and IMPDH activity in OLT and kidney transplant recipients. Understanding the kinetic and dynamic aspects of MPA in OLT patients is important since alterations in serum concentrations due to modification of EHC could adversely affect the outcomes of patients receiving OLT. Changes in protein binding post transplant may also impact on efficacy and/or toxicity. Finally, with the extensive use of MMF in UNC's transplant program and other programs, characterization of the pharmacokinetics and pharmacodynamics of MPA on OLT patients may significantlly improve outcomes in these patients.